File and Directory Change Notification

File and Directory Change Notification

[Joseph D. Wagner]

The docs are a little sketchy on this one.

Using fcntl(), can you get change notifications on specific files, or just 
directories?

TIA.

Joseph D. Wagner

Re: File and Directory Change Notification

[Jamie Lokier]

Joseph D. Wagner wrote:
> The docs are a little sketchy on this one.
> 
> Using fcntl(), can you get change notifications on specific files, or just 
> directories?

You can use F_NOTIFY to get notifications of all files in a directory
(the whole directory, and only for operations via this path in case of
hard links), and F_SETLEASE to get notifications of other processes
opening a file.

There's no way to be notified of every individual write to a single
file, but that isn't typically useful anyway.

What's missing from leases is the ability to block until the final
close of a file (and block further opens in the meantime), and for
F_RDLCK leases to fail/block when the file is open for writing
(there's a FIXME in the kernel lease code for that, but nobody has
fixed it).

I'm not sure why exclusive leases are always allowed but mandatory
locking isn't by default.  They seem to offer similar DOS possibilities.

-- Jamie

Re: File and Directory Change Notification

[Joseph D. Wagner]

On Wednesday, November 26, 2003 10:15 am, Jamie Lokier wrote:
> What's missing from leases is the ability to block until the final
> close of a file (and block further opens in the meantime), and for
> F_RDLCK leases to fail/block when the file is open for writing
> (there's a FIXME in the kernel lease code for that, but nobody has
> fixed it).

Could you direct me to where that fixme is (file and function name)?

Thanks for your clearification.

Joseph D. Wagner

Re: File and Directory Change Notification

[Jamie Lokier]

Joseph D. Wagner wrote:
> On Wednesday, November 26, 2003 10:15 am, Jamie Lokier wrote:
> > What's missing from leases is the ability to block until the final
> > close of a file (and block further opens in the meantime), and for
> > F_RDLCK leases to fail/block when the file is open for writing
> > (there's a FIXME in the kernel lease code for that, but nobody has
> > fixed it).
> 
> Could you direct me to where that fixme is (file and function name)?

fcntl_setlease in fs/locks.c

-- Jamie

Re: File and Directory Change Notification

[Joseph D. Wagner]

On Wednesday, November 26, 2003 10:15 am, Jamie Lokier wrote:
> What's missing from leases is the ability to block until the final
> close of a file (and block further opens in the meantime), and for
> F_RDLCK leases to fail/block when the file is open for writing
> (there's a FIXME in the kernel lease code for that, but nobody has
> fixed it).

Hmm.  Hasn't been fixed in 2.6.0-test10 either.

Re: File and Directory Change Notification

[Matthew Wilcox]

On Wed, Nov 26, 2003 at 12:01:54AM +0600, Joseph D. Wagner wrote:
> Hmm.  Hasn't been fixed in 2.6.0-test10 either.

Your patch is welcome.

-- 
"It's not Hollywood.  War is real, war is primarily not about defeat or
victory, it is about death.  I've seen thousands and thousands of dead bodies.
Do you think I want to have an academic debate on this subject?" -- Robert Fisk

Re: File and Directory Change Notification

[Matthew Wilcox]

On Wed, Nov 26, 2003 at 04:15:48AM +0000, Jamie Lokier wrote:
> I'm not sure why exclusive leases are always allowed but mandatory
> locking isn't by default.  They seem to offer similar DOS possibilities.

Exclusive leases will get broken by the kernel if the application doesn't
cooperate by releasing the lease.

-- 
"It's not Hollywood.  War is real, war is primarily not about defeat or
victory, it is about death.  I've seen thousands and thousands of dead bodies.
Do you think I want to have an academic debate on this subject?" -- Robert Fisk